A growing market - natural fibres for technical textiles

Technical textiles using natural fibres are becoming a more realistic option. John Williams, Principal Lecturer of the TEAM Research Group at De Montfort University, Leicester, UK, explains

World fibre production in 2007 was approximately 80Mt (The Fiber Year Book 2008/9). The two major types used in textile production are cotton (38%), to provide comfort, and polyester (44%), used for durability. With a growing global population, increasing wealth and the need for sustainable textiles, focus is shifting to new and emerging fibres as well as local production of established fibres using new processing methods to increase yield or quality.

Historically, bast fibres have been extracted from the soft outer stem (cambium) of dicotyledonous plants such as hemp and flax by allowing by allowing them to ‘dew ret’ during harvesting. The cut stems are laid on the ground where microbial action degrades it. The uncertainty of the weather means that the process is impossible to control and the quality of the end product is variable. Research by the Textile Engineering And Materials (TEAM) Research Group at De Montfort University, Leicester, UK, has been directed towards improving the agronomy and optimisation of the harvest time to produce a consistent source of fibre.

The viability of growing plants, such as flax, is marginal for the farmer who sells partially retted straw to a centralised processing unit, and transportation costs erode any profit margins. Local ‘farm based’ processing is being investigated by TEAM so that high quality fibre can be produced and sold alongside the waste products, such as shive, into other non-textile markets such as animal bedding and biofuels.

Natural reinforced composites

The use of natural fibres in composites goes back 5,000 years when straw was used to reinforce mud-based bricks.

The composites industry regards glass as its main fibre by volume, although aramids and carbon are used in specialist applications. The increasing drive for sustainability has sparked a renewed interest in natural fibres. French and German applications in areas such as the automotive industry are well established and the necessary supply chains are in place due to subsidies paid to farmers. In the UK, the lack of a significant supply chain presents a serious barrier to expanding the use of natural fibres in composites. The work at De Montfort aims to provide an embryonic supply chain encompassing growers, processing capacity and end-users.

Natural fibres are divided into two main groups (i) organic fibres comprising vegetable (cellulosic) and animal (protein) matter, and (ii) mineral fibres such as asbestos. While many varieties exist, those of most interest have a structural role in nature and are often chosen because of their availability. Vegetable fibres are the most commonly used in composites. The advantages the lower density natural cellulosic fibres have to offer in composites, especially for non-structural applications, are attracting interest from industry. Composite materials reinforced with natural fibres are also being developed, their applications are limited but they are expected to play a role in construction and transport.

With good specific mechanical properties, natural fibres offer a renewable raw material that can reinforce composites. While they may exhibit a degree of flame resistance, composites can be engineered to be totally combustible and do not require landfill at their end-of-life. Biodegradation is also possible for natural fibre-reinforced composites.

A suitable natural-based resin will allow degradedation by microorganisms such as bacteria. This produces water, carbon dioxide and/or methane, and possibly non-toxic by-products. Natural fibres are considered to be carbon dioxide neutral since their combustion or biodegradation only produces a quantity of carbon dioxide equal to that absorbed by the plant during growth.

The performance of fibre-reinforced composites is strongly influenced by the orientation of the fibres within the resin matrix. Unidirectional reinforcement gives composites anisotropic properties, which provide enhanced performance, finding use in industries such as aerospace and high-performance engineering. These composites require continuous filament and cannot therefore be addressed with natural fibres.

An alternative approach is the use of chopped fibres. This gives a random orientation and results in more isotropic materials. While natural fibres are suitable for a chopped fibre composites, they cannot be used where continuous filaments are required. However, converting fibre into yarns and then weaving it into fabrics provides a potential solution. These woven systems, produced on standard looms, are an alternative to a uniaxial preform but the presence of more crimp (cross over points) creates weak spots and reduces performance. Deformation to complex shapes is also affected by the design of the woven fabric, however, with modern weaving and knitting techniques, it is possible to optimise the design to conform to a shape and provide reinforcement where required.

Sting in the tale

Recent research has investigated the growth and extraction of fibre from specific strains of stinging nettles (Urtica dioica). Used in the middle ages (for cloth and food) and latterly during the Second World War for uniforms and sandbags, interest is increasing in the commercial cultivation of this traditional British weed, which attracts a diverse range of wildlife. In conjunction with Camira Fabrics of Dewsbury, a wool/nettle fabric has been produced for industrial upholstery, launched under the trade name STINGplus at the end of 2008. Incorporating nettle fibre into a wool fabric enhances the fire retardant properties of the blend, an advantage when used in industrial applications, particularly transport.